<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>参考答案</title>
</head>
<style>
    div {
        position: absolute;
        width: 200px;
        height: 200px;
        top: 50%;
        left: 50%;
        margin-left: -100px;
        margin-top: -100px;
        z-index: 1000;
        background-color: #bfa;
    }
</style>

<body>
    <div></div>
</body>

</html>